The president, speaking to thousands at a Tucson memorial for Saturday's shooting victims, says our arguments should be 'worthy of those we have lost.'

Reporting from Tucson and Los Angeles — President Obama, facing the challenge of consoling Arizona and uniting the nation, urged Americans on Wednesday not to point fingers of blame but to "expand our moral imaginations" and to "sharpen our instincts for empathy."

Speaking at a memorial for victims of the Tucson shooting rampage that left six people dead and 13 wounded, including Rep. Gabrielle Giffords (D-Ariz.), the president said the gunman's motives were shrouded in mystery.

"The truth is that none of us can know exactly what triggered this vicious attack. None of us can know with any certainty what might have stopped these shots from being fired, or what thoughts lurked in the inner recesses of a violent man's mind," Obama told a boisterous overflow crowd at the service, held at the University of Arizona's McKale Memorial Center.

"Yes, we have to examine all the facts behind this tragedy. We cannot and will not be passive in the face of such violence.... But what we can't do is use this tragedy as one more occasion to turn on each other."

As soon as he arrived in Tucson, Obama and his wife, Michelle, traveled to University Medical Center to visit victims of the attack. They met privately for about 10 minutes with Giffords and her husband, astronaut Mark E. Kelly, the White House said.

At the memorial, Obama departed from his prepared text to announce — with Kelly's permission, the president said — that Giffords had opened her eyes for the first time shortly before the service.

"She knows we are here and she knows we love her," Obama said. The crowd erupted.

Kelly, who was in the audience, received a hug from Michelle Obama.

More than 13,000 people packed into the auditorium, the university said, for a service that was at turns somber and sorrowful, defiant and triumphant. Another 13,000 who couldn't fit inside watched from a nearby football stadium.

The crowd — students, retirees, parents and children, Obama supporters, Obama opponents, people who knew the shooting victims and many more who did not — had waited more than 12 hours to get inside. Lines snaked for miles.

Joe Watkins, 50, a trial lawyer who attended with his wife and a co-worker, said he was "sick to death of the negativity that's been thrown around the past few campaigns."

"Now everybody on both sides of the aisle has stepped back and said, 'We have to think.' But will it last?" he asked.

As the ceremony began, an elderly woman unfurled a homemade sign that read: "We will heal."

The event came four days after a gunman, whom police identified as 22-year-old Jared Lee Loughner, fired 31 shots outside a Tucson Safeway. The dead included Arizona's chief federal judge, John M. Roll.

Authorities said Loughner's primary goal appeared to have been to assassinate Giffords. She had been hosting a Congress on Your Corner event, which the president called a "quintessentially American scene" — a congressional representative listening to constituents.

The shooting plunged much of Arizona and Washington into sadness, but also brought a renewed focus on incivility and violent imagery in politics.

Obama confronted that issue, saying that although debates over gun control or mental healthcare were important and proper, Americans should debate "in a way that heals, not [in] a way that wounds."

Incivility did not cause the attack, he said, but added that our debates should be "worthy of those we have lost" — not conducted "on the usual plane of politics and point-scoring and pettiness that drifts away with the next news cycle."

Obama seemed to meld his customary austerity with an emotional accounting of the attack's toll. One by one, he told the stories of those killed — a snowbird who often knitted under a tree; a woman married to her high school sweetheart for 50 years; a retired construction worker who died while shielding his wife with his body; a federal judge on his way home from Mass; a Giffords aide who helped constituents.

The president seemed particularly moved when he recounted the life of the youngest victim, 9-year-old Christina Taylor Green, who was about the same age as Obama's younger daughter, Sasha.

Christina was an A student, he noted, a member of her student council and the only girl on her youth baseball team. Born on Sept. 11, 2001, she was featured in a book about babies born on that tragic day, he said, with her photographs accompanied by text describing hopes for her life.

Those hopes included: "I hope you jump in rain puddles."

"I want America to be as good as she imagined it," Obama said. "… If there are rain puddles in heaven, Christina is jumping in them today."

Obama's rare display of emotion helped him reach out to grieving listeners, presidential historian Doris Kearns Goodwin said. But he had a greater responsibility to fulfill, she said — to take stock of the moment and offer meaning and inspiration.